1. Field of the Invention
This invention relates to an optical fiber holding for holding an optical fiber for an optical fiber communication system, an optical dispersion-equalizer for compensating wavelength dispersion of a number of optical signals propagating through the optical fiber, and a method of manufacturing the optical fiber holding device.
2. Description of Related Art
FIG. 14 is a structural drawing of a conventional optical fiber holding device.
Referring to FIG. 14, reference numeral 1 denotes an optical fiber made of a core and a clad; 2 a grating which is formed at a part of the core of the optical fiber and reflects an optical signal of a number of wave lengths; 3 heaters which are made of a thin film for heating the grating to a predetermined temperature distribution; and 4 a substrate which is, for instance, made of quartz and on which the heater 2 is mounted. Furthermore, the grating 2 is used for compensating the wavelength dispersion of a number of optical signals propagated through the optical fiber 1.
As shown in FIG. 14, the optical fiber 1 is directly mounted on the heater 3 which is made of the thin film. Therefore, in order to heat the grating 2 to the predetermined temperature distribution, it is necessary to carry out an accurate positioning of the grating 2 in the longitudinal and the transverse directions, and to mount it on the heater 3. The positioning of the grating 2 with respect to the heater 3 in the longitudinal direction should be made such that a positioning marker (not shown) for the grating 2 provided on the surface of the optical fiber at the both ends of the grating 2 is aligned with the both ends of the heater 3. Similarly, the positioning of the grating 2 with respect to the heater 3 in the transverse direction should be made such that the center of the heater 3 is put on an axial center of the grating 2 in the longitudinal direction.
The positioning of the grating 2 with respect to the heater 3 in the longitudinal and the transverse directions is made on a microscope, while conforming the position of the grating 2, with a tension added to the optical fiber 1. However, since the diameter of the optical fiber is 125 μm or so (60 μm, for instance), accurate positioning and mounting of the grating 2 on the heater 3 is difficult and takes a lot of time What is worse, the both ends of the optical fiber 1 of the grating 2 are only secured after the position, and any fixations are not had to the grating 2. Therefore, there can be a possibility that dislocation of the grating 2 with respect to the heater 3 would be occurred.
FIG. 15 is a structural drawing of another conventional optical fiber holding device.
Referring to FIG. 15, reference numeral 5 denotes an adhesive for securing the optical fiber 1 having a grating 2 to the substrate 4. The other structure other than the adhesive 5 is the same as that in FIG. 14. As shown in FIG. 15, the grating 2 of the optical fiber 1 is mounted on the heater 3 in the same manner as the optical fiber holding device in FIG. 14. Then, an adhesive is coated on the heater 3 so as to secure the optical fiber 1 to the substrate 4. Here, adhesive paste or resin may be used instead of the adhesive 5.
In this case, when the adhesive 5 is coated, the grating 2 displaces due to viscosity of the adhesive 5, leading to displacement of the grating with respect to the heater 3. On the other hand, when the adhesive 5 is cured, the grating 2 also displaces due to the volume change caused by curing of the adhesive 5, resulting in displacement of the grating 2 with respect to the heater 3. At that moment, a stress resulting from the volume change when the adhesive 5 is cured is applied to the optical fiber 1, which deforms a shape of the core and occurs a double reflection, with the result that a polarization mode dispersion characteristic will be degraded.
The polarization mode dispersion (PMD) will be briefly described.
Generally, an optical signal in a single mode optical fiber is present as a propagation mode including two orthogonal polarization modes. But, the two orthogonal polarization modes undergo a change in their characteristics by any disturbances and thus the normal propagation mode suffers from degradation by the double reflection. It is called as a polarization mode dispersion. As a result of the disturbances, the double reflection is occurred. Typical disturbances are listed, i.e., heterogeneity of the core shape, heterogeneity of compositions in the core, deformation of the core shape due to a stress application, and heterogeneity of temperature in the core.
Such a conventional optical fiber holding device as shown in FIG. 14 gives rise to difficulties in accurately positioning the grating 2 on the heater 3 and may cause dislocation of the grating 2 with respect to the heater 3, even through the grating 2 has already mounted on the heater 3.
Further, in the optical fiber holding device shown in FIG. 15, when the adhesive 5 is coated, viscosity of the adhesive 5 causes dislocation of the grating 2 with respect to the heater 3 and, when the adhesive 5 is cured, the volume change due to curing causes the same.
Moreover, in the optical fiber holding device shown in FIG. 15, the stress resulting from the volume change when the adhesive 5 is cured is applied to the optical fiber 1, suffering from degradation of the polarization mode dispersion characteristic.